The challenge of rooftop gardens

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Vegetation shaped in Expo mascot Haibao dots the roof of the Yangpu government building.

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BEAUTIFUL hanging gardens, walls of greenery, vertical vegetable gardens with rip fruit, and rooftop gardens with streams and shade are among the most delightful aspects of the World Expo 2010.

Living green plants show up in unlikely places, and there are even transparent walls of hydroponically grown plants.

They are not only beautiful and soothing for the eyes, they also help cool, insulate, purify the air, reduce dust and noise. They reduce carbon dioxide while producing oxygen.

But creative gardening is showing up around Shanghai as well. The number of green rooftops has expanded rapidly over 10 years in the city.

Though not as spectacular and creative as the Expo pavilion roof gardens, "three-dimensional gardening" is not new in Shanghai.

Green walls, green spaces around buildings, green elevated road edges, as well as green roofs have improved the city, making it more attractive and healthy.

Roof greenery, which is relatively new, covered around 900,000 square meters of roofs in Shanghai by the end of 2009, according to Li Li, senior horticulturist of the Shanghai Landscaping and City Appearance Bureau.

Rooftops are definitely greener in Jing'an, Yangpu and Minhang districts. Rooftop greenery covers about 120,000 square meters in Jing'an District, 27,565 square meters in Yangpu District and 400,000 square meters in Minhang District.

"The earliest green roof in Shanghai go back to the early 20th century when hotels decorated their roofs to improve the setting," says Li. "But little progress was made for dozens of years."

In the 1980s, a few business areas with garden roofs appeared, but not until 1998 did three-dimensional gardening really expand, with government help.

"Shanghai is a big city with a huge population and limited land," says Li. "Expanding greenery in three dimensions will certainly improve the environment."

In addition to benefits such as pollution and noise reduction, roof gardens reduce indoor temperatures, combat the "heat island" effect, prolong roof life and retain rainwater, reducing runoff.

Research shows that roof with green cover lasts an average of 40 to 50 years, while that of a bare roof exposed to the elements is only 25 years, according to research cited by Li.

Cool off

A study in Minhang District shows that indoor summer temperatures of buildings with green roofs are lower by 3-5 degrees Celsius than those of exposed roofs on days of more than 34 degrees Celsius.

If six percent of the roofs in a city are covered by vegetation, the "heat-island" effect is expected to drop by 1-2 degrees Celsius, contributing to the reduction of 1.5-2.1 trillion tons of greenhouse gas emissions a year.

Green rooftops help during frequent storms in Shanghai's summer. Studies show they can retain up to 75 percent of the rainwater in given storm, gradually releasing it back into the atmosphere through condensation and transpiration, while retaining air pollutants in the soil.

"Roof greenery is definitely suitable for Shanghai, but technical concerns, shortage of funds and low awareness among residents have slowed development for years," says Li, the greenery official.

Roof greenery is technically more difficult to carry out due to safety concerns like load-bearing, and waterproofing. Most new buildings today have good load-bearing construction and water-proofing facilities, but most old houses need reinforcement and water-proofing layers.

Large investments have been made for roof gardens in public areas in some districts, while most residential and private buildings still have not gone.

"Many residents reject green roofs as they think that it only benefits the top-floor residents while adding property fees for everyone," says Li.

To avoid problems with residents and ensure safety, the government is first planting gardens on roofs of public buildings, such as government offices, schools and hospitals.

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